I came across a new and free SW for the dBm calculation.
Brief:application software comprises five independent calculation tools:• dBmCalculator: This tool helps to add or subtract power levels expressed in
units of dBm or watts. (An ideal 3 dB-combiner is taken into account.)
• VoltageCalculator: This tool helps to mathematically add or subtract voltage
levels.
• UnitConverter: This tool converts power units and voltage units.
• RatioConverter: This tool converts a linear power or voltage ratio to dB.
• VSWRConverter: This tool converts between different reflection units such as
VSWR and return loss

More information on its user guide & exe are available in
http://www2.rohde-schwarz.com/en/service_and_support/Downloads/Application_Notes/?type=20&downid=5499